The present invention relates to vacuum gauges and, more particularly, to cold cathode ionisation vacuum gauges.
Cold cathode ionisation gauges for measuring vacuum, sometimes referred to as xe2x80x9cPenningxe2x80x9d gauges are well known in the vacuum art. They usually comprise an anode and one or more cathodes. In use, a large potential difference is applied between the anode and the cathode or cathodes and a substantial magnetic field is applied by a permanent magnet in the area between the electrodes. The anode and the cathode or cathodes are held in a predetermined configuration relative to each other by means of a vacuum feedthrough which isolates the electrodes within the gauge from the atmosphere outside.
In use, when the gauge is subjected to the vacuum to be measured a discharge will be formed between each cathode and the anode in which electrons are accelerated towards the anode by the electric field. However, the action of the magnetic field applied by the permanent magnet causes the electrons to adopt a very long, non-linear trajectory, for example, a helical trajectory, before striking the anode. As such, the probability of ionisation of gas molecules present in the vacuum by collision with the electrons is much higher even at low pressures. Electrons formed by the collisions are attracted by the anode to produce a current in an external circuit, the size of which is related to the molecular weight and density of the gas at a given temperature and therefore to the level of vacuum.
In gauges of the inverted magnetron type, the cathode commonly has associated with it means to define a region or cell within it in which the discharge is confined and the electron current to the external circuit is generated.
However, in such gauges problems are sometimes encountered in the initiation of the discharge (strike or start-up) of the gauge at very low pressures, for example less than 1xc3x9710 to xe2x88x927 mbar. In particular, the gauge may take several minutes or even hours to strike because of the low probability of an ionising event occurring.
In GB Patent Publication No. 2256310 a vacuum gauge of the ionisation type includes a gauge head having an outer cathode sleeve with an inlet at one end for communication with a vacuum and a pair of cathode discs positioned substantially radially within the cathode sleeve and defining between them an ion collection region or cell. The discs both have a central aperture and an anode is positioned within the cathode sleeve and within the central apertures of the discs. The strike time of this known vacuum gauge is improved by the provision on the surface of at least one of the apertures formed on the cathode discs of a sharp point or edge which is presented towards the anode.
It is an aim of the present invention to provide a vacuum gauge of the cold cathode type having an electrically operated ignition device positioned in the vicinity of the discharge cell to improve the strike time by providing a source of electrons, the ignition device being protected and controlled using an auxiliary pressure measuring device.
According to the present invention, a vacuum gauge of the cold cathode type has a gauge head comprising an outer cathode sleeve having an inlet at one end for communication with a vacuum to be measured, a cathode discharge cell located within the sleeve, an anode, a portion at least of which is located within the cathode discharge cell and in which an electrically operated ignition device is located adjacent the cathode discharge cell which device is protected from being operated at high pressure by an auxiliary pressure sensor. The ignition device may be an electrically heated filament or a field-emitting electrode.
Likewise, the auxiliary pressure sensor or pressure measuring device may be a thermal conductivity sensor, a gas friction sensor or a diaphragm sensor. When the ignition device is an electrically heated filament said filament itself may be used as the auxiliary pressure sensor.
In a preferred embodiment the auxiliary pressure sensor is integral within the gauge head.
An embodiment of the invention will now be described, by way of example, reference being made to the accompanying figure described below.